1. Field of the Invention
The present invention relates to digital graphics systems. More specifically, the present invention relates to methods and circuits for de-interlacing digital pictures.
2. Discussion of Related Art
Analog video displays such as cathode ray tubes (CRTs) dominate the video display market. Thus, most electronic devices that require video displays, such as computers and digital video disk players, output analog video signals. As is well known in the art, an analog video display sequentially reproduces a large number of still images to give the illusion of full motion video.
Analog video signals for display on most consumer video systems are interlaced. Interlaced video signals contain data for every other line of a de-interlaced picture. Therefore, every other line is displayed each time the screen is refreshed. The portion of the interlaced video signal containing the odd scanlines (rows) of a picture is called the odd field and the portion of the interlaced video signal containing the even scanlines of a picture is called the even field.
For computer applications, a process called xe2x80x9cprogressive scanxe2x80x9d is used to refresh the screen. In progressive scan, each line in a video signal is displayed in sequence, from the top of the screen to the bottom of the screen. As a result, each picture displayed in progressive scan is a full picture, rather than the half pictures displayed with interlaced video. For this reason, this type of picture display is also called xe2x80x9cnon-interlacedxe2x80x9d video.
Color video signals contain luminance and chrominance information. Luminance is that portion of video corresponding to brightness value. Thus, luminance is the grayscale brightness value of a black-and-white picture. Chrominance is that portion of video that corresponds to color values and includes information about hue and saturation. Color video signals may be expressed in terms of a red component, a green component, and a blue component. Thus, luminance and chrominance information may be converted into a red, green and blue component. Luminance may be approximated by a weighted average of the red, green and blue components. In one well-known scheme, luminance is approximated by the equation: 0.30*red component+0.59*green component+0.11*blue component.
To create a digital display from an interlaced video signal, the interlaced video signal is digitized to define the pixels of a digital display. Because the video is interlaced, each field digitized will contain data for only half of the digital display. The half-picture data contained within the digitized fields of the interlaced video are usually processed into a full (de-interlaced) picture for digital display to improve picture quality. Current methods of de-interlacing video include BOB and weave de-interlacing.
FIG. 1 illustrates a portion of one field of a digitized video signal. FIG. 1 includes fields F(Z). Fifteen pixels of field F(Z) are shown. Pixels containing digitized data from the interlaced video signal are shown with solid outlines. Pixels requiring interpolated data are shown with dashed outlines. For clarity, pixels in field F(Z) are identified using a 3 dimensional coordinate system. As shown in FIG. 1, pixel P(X,Y,Z) is in the upper left corner of the portion of field F(Z). Pixel P(X+2,Y+1,Z) is in the center of the portion of field F(Z) shown in FIG. 1. Pixel P(X+4,Y+2,Z) is in the lower right corner of the portion of field F(Z) shown in FIG. 1.
BOB de-interlacing repeats the pixel data corresponding to each data-containing row. Thus, de-interlacing field F(Z) of FIG. 1 sets pixel P(X+C,Y+1,Z) equal to pixel P(X+C,Y,Z), where C is the column of the pixel, beginning with C=0. For example, for C=1, P(X+1,Y+1,Z) is set equal to pixel P(X+1,Y,Z). BOB de-interlacing induces a distortion into the de-interlaced picture. This distortion is especially apparent when pictures contain diagonal lines, where the BOB de-interlacing produces a stair-step effect rather than a smooth diagonal line.
FIG. 2 illustrates a portion of three adjoining fields of a digitized video signal. FIG. 2 includes successive fields F(Zxe2x88x921), F(Z), and F(Z+1). One pixel in each of fields F(Zxe2x88x921), F(Z), and F(Z+1) is shown. Pixels containing digitized data from the interlaced video signal are shown with solid outlines. The pixel requiring interpolated data is shown with a dashed outline. Field F(Zxe2x88x921) precedes field F(Z) which precedes field F(Z+1). One row and column is shown for each of fields F(Zxe2x88x921), F(Z), and F(Z+1). For clarity, the pixel in each field is identified using a 3 dimensional coordinate system. As shown in FIG. 2, pixel P(X,Y,Z+1) is in the center of the portion of field F(Z+1) shown in FIG. 2. Pixel P(X,Y,Z) is in the center of the portion of field F(Z) shown in FIG. 2. Pixel P(X,Y,Zxe2x88x921) is in the center of the portion of field F(Zxe2x88x921) shown in FIG. 2.
Weave de-interlacing uses the preceding field and the subsequent field to interpolate the unknown pixels in each field. Specifically, weave de-interlacing averages the pixel data in the preceding field with the pixel data in subsequent field. Thus, de-interlacing field F(Z) of FIG. 2 sets pixel P(X,Y,Z) equal to the average of pixel P(X,Y,Z+1) and pixel P(X,Y,Zxe2x88x921). Weave de-interlacing induces a distortion into the de-interlaced picture. This distortion is an elongation effect apparent when the digitized interlaced video contains motion.
It would be desirable to de-interlace digitized interlaced video with minimal distortion.
The present invention de-interlaces digitized fields (pictures) in a manner that accounts for the motion of objects in the fields. In accordance with an embodiment of the present invention, a de-interlaced picture is created from pixel data in three adjacent fields. The center field of the three adjacent fields is used as the basis for the de-interlaced picture. The pixels to be interpolated in the de-interlaced picture are calculated from a combination of percent differences of pixels located in rows adjoining each pixel to be interpolated in the center field (i.e., adjoining rows in the same field) and percent differences of pixels located in the rows of each pixel to be interpolated in the preceding and subsequent fields (i.e., adjoining columns in the adjacent fields). Luminance information effectively preserves the motion information in a video sequence. Thus, another embodiment of the present invention utilizes luminance information for the pixels used in the interpolation to account for motion.
In another embodiment of the present invention, an inverse weighted average of the minimum same-field percent difference and the minimum adjoining-field percent difference is used. Another embodiment of the present invention adds a threshold test preferring center pixels having a percent difference below the threshold to minimize soft noise in the calculation. Center pixels are pixels in the same column but adjoining rows of the same-field and pixels in the same row and same column in adjacent fields. When the center pixels have a percent difference above the threshold, this embodiment of the present invention picks one of the pixel pairs having a percent difference below the threshold. Pixel pairs are the opposing pixels used to calculate a percent difference. If all of the pixel pairs have percent differences above the threshold, then the pixel pair having the minimum percent difference is chosen.